Electrical fuses (eFuses) are used in the semiconductor industry to implement array redundancy, field programmable arrays, analog component trimming circuits, and chip identification circuits. Once programmed, the programmed state of an electrical fuse does not revert to the original state on its own, that is, the programmed state of the fuse is not reversible. For this reason, electrical fuses are called One-Time-Programmable (OTP) memory elements.
The mechanism for programming an electrical fuse is electromigration of a metal semiconductor alloy induced by an applied electrical field and an elevated temperature on a portion of the electrical fuse structure. The metal semiconductor alloy is electromigrated under these conditions from the portion of the electrical fuse structure, thereby increasing the resistance of the electrical fuse structure. The rate and extent of electromigration during programming of an electrical fuse is dependent on the temperature and the current density at the electromigrated portion.
An electrical fuse typically comprises an anode, a cathode, and a fuselink. The fuselink is a narrow strip of a conductive material adjoining the anode and cathode. During programming of the electrical fuse, a positive voltage bias is applied to the anode and a negative voltage bias is applied to the cathode. As electrical current flows through the fuselink having a narrow cross-sectional area, the temperature of the fuselink is elevated. A high current density combined with the elevated temperature at the fuselink facilitates electromigration of the conductive material, which may comprise a metal silicide.
A typical prior art electrical fuse employs a stack of a gate dielectric, a polysilicon layer, and a metal silicide layer. Under electrical bias through the electrical fuse, the metal silicide layer provides an initial current path since a typical metal silicide material has a conductivity at least one order of magnitude greater than the conductivity of even the most heavily doped polysilicon material. As the metal silicide material electromigrates, the electrical current path formed by the initial metal silicide layer is broken. Further, the high temperature that the metal silicide layer generated prior to completion of electromigration contributes to dopant electromigration in the polysilicon layer underneath, causing depletion of the dopants in the polysilicon layer in a programmed prior art electrical fuse. A programmed electrical fuse attains a high enough resistance so that a sensing circuit may detect the programmed electrical fuse as such. Thus, the prior art electrical fuse containing a vertically abutting stack of the gate dielectric, the polysilicon layer, and the metal silicide layer provides an OTP memory element without introducing any additional mask level or any extra processing steps.
However, continuous advances in the semiconductor technology oftentimes require changes in the material employed in semiconductor structures. Of particular relevance is the advent of a metal gate electrode, which, in addition to the gate dielectric, a polysilicon layer, and a metal silicide layer, contains a metal gate layer in a gate stack. Typically, the metal gate layer is employed in conjunction with a high-k gate dielectric material. This is because high gate leakage current of nitrided silicon dioxide and depletion effect of polysilicon gate electrodes limits the performance of conventional silicon oxide based gate electrodes. High performance devices for an equivalent oxide thickness (EOT) less than 1 nm require a high-k gate dielectric material and a metal gate electrode to limit the gate leakage current and provide high on-currents.
The high-k gate dielectric materials refer to dielectric metal oxides or dielectric metal silicates having a dielectric constant that is greater than the dielectric constant of silicon oxide of 3.9 and capable of withstanding relatively high temperatures, e.g., above 600° C., and preferably above 800° C. The metal gate layer may comprise a metal, a metal alloy, or a metal nitride, and typically has an even higher conductivity than the metal silicide.
The presence of the metal gate layer in the metal gate electrode make programming of an electrical fuse containing the metal gate layer extremely difficult. This is because a properly programmed electrical fuse must not contain a high conductivity current path to insure that the resistance of the programmed electrical fuse is sufficiently high. Thus, the introduction of metal gate electrodes into semiconductor devices has a disadvantage side effect of introducing a metal gate structure into an electrical fuse, and making proper programming of the electrical fuse difficult due to the presence of the metal gate layer in the electrical fuse. Degradation of electrical fuse programming characteristics has been confirmed in electrical fuses having a metal gate structure during the course of the research leading to the present invention.
In view of the above, there exists a need for an electrical fuse structure accommodating fabrication of other semiconductor devices employing metal gate electrodes, yet providing good programming characteristics including high post-programming resistance, and methods of manufacturing the same.